MET-2010 PRECISION METEOROLOGICAL THERMO-HYGROMETER

Description

The Model MET-2010 is a complete precision ambient temperature and dew/frost
point measuring system designed for the meteorologist, climatologist, or
atmospheric scientist. It provides a superior degree of accuracy and
reliability, achieving an absolute accuracy of ±
0.05° C in measuring ambient temperature over the
entire ± 50° C range.
Dew point temperatures above 0° C are determined to
an accuracy of ± 0.2° C,
while frost point temperatures down to -70° C have
an accuracy of ± 0.5° C.
The system consists of an aspirated triple-shielded platinum resistance
thermometer and an intelligent, chilled mirror dew/frost point hygrometer
sensor. Above 0° C, the hygrometer tracks the dew
point temperature; below 0° C, it tracks the frost
point temperature.

Ambient temperature is highly variable in both time and space, and a single
measurement reflects a limited space-time domain. Historically, techniques other
than direct measurement of ambient temperature have provided support for the
global warming issue, but precise knowledge of atmospheric ambient temperatures
is a fundamental component to understanding the Earth's climatic mechanisms. As
we continue collecting ambient temperature data for the climatological database,
improvements in the basic measurement are required. While some of the problems
of measuring ambient temperature cannot be easily eliminated, basic instrumental
and observational errors in climatic temperature measurementscan be
reduced using the MET-2010.

Specially plated hygrometer
mirror to prevent corrosion and readily accessible for cleaning through a
control box port. No need to open the control box to check the mirror, a plus
for maintenance in inclement weather

Automatic detection and
indication of fan failure

Precision platinum resistance
thermometer

Thermometer triple radiation
shields

MET-2010 system

Applications

The MET-2010 offers nearly an order of magnitude more precision than existing
ambient temperature measuring systems, making it ideally suited for:

Professional grade
meteorological stations

Aviation weather system
upgrades

Field accuracy checks of other
field sensors

Industrial calibration
laboratories

Agricultural and global climate
change research

Mechanical Configuration

The MET-2010 electronics are packaged in a weatherproof
NEMA-type PVC enclosure. The thermometer radiation shields are manufactured of
aluminum with an electrostatically-applied white powder coating for long life.
Electrical and mechanical components are designed to operate over a temperature
span of ± 50° C; reduced
aspirator motor life can be expected when system installation temperatures are
below -50° C due to bearing lubricant degradation.
The system typically mounts two meters above the ground to a customer tower or
support and aimed into the prevailing wind.

The ambient temperature measuring system employs a modification of the
four-wire Standard Platinum Resistance Thermometer (SPRT) design. The
modifications preserve the fundamental features of the SPRT, including minimum
stem conduction error, strain-free mounting of the platinum element, and
four-wire Kelvin readout. Features not required by the meteorological
application are eliminated, bringing the SPRT within the range of affordability
for field measurements. The MET-2010's 100 Ohm platinum resistance thermometer
element is calibrated after packaging in a NIST-traceable temperature bath (Hart
Scientific Model 7025 high precision bath). Alone, the thermometer provides an
uncertainty of ± 0.028°
C. The thermometer is then mounted in an aspirated, triple-walled aluminum
radiation shield that has a worst case, full-sun, radiation error of less than
+0.005° C. Readout of the 100W
PRT element is achieved through the use of a 20-bit analog-to-digital bridge
completion circuit that excites the resistance element with a current of 400 µAac.
This results in a self-heating error of less than 0.0016°
C and elimination of thermoelectric voltages.

Several potential sources of error should be considered in designing an
ambient temperature measuring system that strives to offer absolute accuracy of ±
0.05° C or better. The major contributors to the
uncertainty analysis in a instrument of this type are the solar radiation
shielding error, the thermometer stem conduction error, and the PRT self-heating
error. The worst-case MET-2010 errors from these sources are +0.005°
C, ± 0.00003° C and
+0.0016° C, respectively. A variety of other random
RMS errors include the calibration limits of the thermometer bath (±
0.028° C), reference resistor drift errors (±
0.00607), amplifier gain errors (± 0.00729°
C), and the linearity error (± 0.0045°
C). When combined, these errors produce a worst-case uncertainty of:

E = Erms + Eheating + Esol

= 0.00973 + 0.0016 + 0.005

= 0.016 °C

Although the RMS uncertainty analysis for all components of the system is
0.016° C, the MET-2010 is conservatively rated at an
accuracy of ± 0.05° C.

Digital Output Data Format

The RS-232 serial digital output is used to fully cover the resolution and
precision of measurements. The output stream is similar to other automated
surface observation system thermohygrometers (e.g. the TSL Model 1800). The
MET-2010 responds to a "T" string at the serial port with a constant
length record. This includes a T echo; then two, five-digit-long right-justified
temperature and dew point values (selectable as ° C
or ° F via an on-board jumper); followed by a mirror
status and blower status; terminating with CR and LF characters. An example of a
MET-2010 fixed length output record follows:

T -0.85 22.54 DIRTY=0 FANFAIL=0

Internal CPU Operation

The Met-2010 is controlled by an embedded microprocessor CPU that serves
several functions:

At power up, the processor
monitors a heating cycle whereby the chilled mirror is heated to approximately
48° C and a measurement of the dry mirror reflected
signal is obtained.

Measurements are compared to a
reference signal to determine if the mirror is in need of cleaning. A test
heating cycle is automatically performed once daily at midnight UTC, as
determined by an on-board clock/calendar.

While monitoring the serial port
for commands, continuous measurements of the dew point and ambient temperature
are made by the ADC.

Upon receipt of the query command
(T1), the CPU outputs a single data record, containing the dew point
temperature, ambient temperature, and mirror condition of "CLEAN" or
"DIRTY".

The CPU performs digital
filtering of the data to minimize measurement noise. It performs a quadratic fit
to within ± 0.001° C of
ITS-90 for converting the SPRT resistance values to degrees centigrade.

The D/A converter's analog output
indicates the prevailing dew point and ambient temperatures.

Electrical Connections

Two foil-shielded 10' (3m) long cables are supplied with pig tails for
interface to the user's junction box:

Cable runs are possible up to 50' via a user cable splice or junction box.
Both DCE and DTE RS-232 serial interfaces can be interfaced.

MET-2010 software interface

Available Options

Spare aspiration blower

MS-Windows 9x/NT of Mac viewing
software

Temperature-only version
(PMT-2005)

RJ-45 interface via 10BaseT to
a LAN

Development History

The MET-2010 was the result of an intensive five-year R&D effort,
sponsored in part by the United States Department of Agriculture, to develop and
field test an improved means of measuring ambient temperature at climatological
monitoring sites. It represents the first major improvement in air temperature
measurement since the introduction in the 1970s of the Gill radiation shield.

The measurement of ambient air temperature is fundamental to the study of
global climate and climate change mechanisms. Assessing the degree of global
warming occurring from the increase of so-called "greenhouse gases"
has been frustrated by variations in long-term temperature records due to
earlier ambient temperature sensors that had calibration drifts. These
variations stemmed from a variety of factors, including changes in the
technology of measurement, methods of exposing the instrument, frequency of data
collection, methods of calibration, and the training and dedication of the
person collecting the data.